Effusive and evolved monogenetic volcanoes: two newly identified (~800 ka) cases near Manizales City, Colombia

Two recently discovered volcanoes (Las Margaritas 1 and 2) located near the city of Manizales in central Colombia (northern Andes) show that monogenetic volcanoes can be both effusive and be fed by evolved compositions, unlike in most other monogenetic fields. This study presents the results of cartographic, petrographic, geochemical, and geochronological analyses. Mapping indicates that the volcanoes are purely effusive, where the first erupted a dome coulée and the second erupted a lava flow. K/Ar dating of the groundmass yielded emplacement ages of 0.77 ± 0.04 and 0.80 ± 0.05 Ma for each volcano. The rocks in both volcanoes contain plagioclase, amphibole, and Fe-Ti oxides as ubiquitous minerals, but only one volcano hosts biotite. The two volcanoes can also be differentiated by the presence of amphibole oxidation rims in one of them. Both volcanoes are andesitic in composition and have a calk-alkaline signature. Trace elements show light rare earth element (LREE) enrichment, and negative Th, Nb, Ta, and Ti anomalies. Overall, the results indicate equilibrium conditions that allowed not only phenocrysts and microphenocrysts to crystallize, but also convection and stagnation processes that allowed zonation and glomerocrysts to form. This, along with the chemical information, indicates subduction characteristics that can be explained by small magma batches breaking off from crustal reservoirs. These effusive monogenetic eruptions are thus associated with efficient degassing during ascent, while compositional evolution is related to relative long-term magma stagnation in the crust.


Introduction
Monogenetic volcanoes are small-volume eruptive centers (≤1 km³) that experience a single eruption in their lifespan (Valentine and Gregg 2008). Typical monogenetic volcanoes are basaltic, and the most common type is a scoria cone (Valentine and Connor 2015). Basaltic scoria cones are formed by the uninterrupted ascent of small magma batches to the surface, usually with scarce or no processes of assimilation and fractional crystallization due to the rapid ascent of magma (McGee and Smith 2016;Murcia and Németh 2020), leading to primitive compositions in some cases (Smith and Németh 2017). Nevertheless, stagnation of magma and processes of assimilation and fractional crystallization can occur, which leads to evolved magmatic compositions ranging up to dacitic and rhyolitic (e.g., Borrero et al. 2017;Ross et al. 2017;Murcia et al. 2019;Sosa-Ceballos et al. 2021;Sánchez-Torres et al. 2019. In general, monogenetic volcanism can be found in any tectonic setting (Németh 2010;Le Corvec et al. 2013;Kereszturi et al. 2014) and monogenetic volcanoes can either form volcanic fields or occur as clusters around composite volcanoes (Schaaf et al. 2005;Cañón-Tapia 2016;Schonwalder-Ángel et al. 2018). A volcanic field consists of an area occupied by various volcanic centers (Valentine and Connor 2015) that can vary from just a few volcanoes (e.g., as at Pijaos field in Colombia; Velandia et al. 2021) to hundreds of centers (e.g., as at Michoacán-Guanajuato field in México; Osorio-Ocampo et al. 2018) and a field can remain active millions of years (Németh 2010).
This study presents the results of mapping, plus petrographic, geochemical, and geochronological analyses of two newly identified volcanoes, named here Las Margaritas 1 and Las Margaritas 2. These analyses are used to determine the landforms as well as their mineralogical and chemical compositions. We use this data to infer the different magmatic processes that occurred during magma ascent, the emplacement ages of the landforms, and therefore their eruptive histories. This work highlights that effusive and evolved monogenetic volcanism is rather common and therefore, it should be considered in the classification schemes of this type of volcanism.

Geological setting
The northwestern margin of South America has been subjected to the collision of oceanic plateaus and island arcs alternating with subduction episodes from the Late Cretaceous to the present day, associated with the interactions between the South American, Caribbean, Farallon/Nazca plates, and the Panamá-Chocó arc (e.g., Kerr and Tarney 2005;Villagómez et al. 2011;Villagómez and Spikings 2013;León et al. 2018;Montes et al. 2019;Zapata et al. 2020). From the late Miocene to the present day, the oblique subduction (re)initiation and collision/subduction of aseismic ridges of the Nazca plate as well as slab flattening have controlled the upper plate architecture and the tectono-magmatic evolution of the continental arc in the northwestern Colombian Andes (e.g., Wagner et al. 2017;Jaramillo et al. 2019;León et al. 2021). The current evidence of this active magmatic arc is the San Diego-Cerro Machín Volcano-Tectonic Province (SCVTP), a volcanic chain that hosts at least 10 polygenetic volcanoes as well as tens of monogenetic edifices grouped into three monogenetic volcanic fields: Pijaos, Villamaría-Termales, and Samaná Monsalve et al. 2019;Murcia et al. 2019;Sánchez-Torres et al. 2019Vargas 2020;Monsalve-Bustamante et al. 2020;Salazar-Muñoz et al. 2021;Velandia et al. 2021) (Fig. 1b). The magmatic affinity of the SCVTP is typically calc-alkaline and compositions vary from basaltic to rhyolitic (e.g., Murcia et al. 2019;Sánchez-Torres et al. 2019Salazar-Muñoz et al. 2021;Velandia et al. 2021). Monogenetic volcanism started at 2 Ma in the Villamaría-Termales field Murcia et al. 2019;Salazar-Muñoz et al. 2021) and is likely still active in the Samaná field Sánchez-Torres et al. 2019.

Volcano distribution and fieldwork
An area of about 120 km 2 was searched for evidence of volcanic constructs. Mapping then focused on the two identified volcanoes for which a digital elevation model (DEM) of 12 * 12 m resolution was used to define their morphology and limits. Field mapping, outcrop descriptions, and sample collection were then carried out at each volcano. Unfortunately, due to tropical weathering and to the thick (3-6 m) younger pyroclastic fall deposits that cover the region, we were able to collect only one single fresh sample per volcano, outcroping in a quarry and in a landslide, respectively. These samples were used for petrography, geochemistry, and dating.

Petrography
Thin sections were prepared for the two samples at TecLab laboratories, Manizales, Colombia, and point counting (~500 points) using a 1-mm grid spacing was used to determine the groundmass/crystal ratio and the proportion of each crystal phase. Following González (2008) and Murcia and Németh (2020), crystal sizes were defined as follows: phenocrysts = >0.5 mm and microphenocrysts = 0.5-0.05 mm. Microlites (˂0.05 mm) were considered part of the groundmass. A petrographic microscope at the Instituto de Investigaciones en Estratigrafía (IIES) at the Universidad de Caldas (Colombia) was used to carry out this analysis.

Whole-rock chemistry
Both samples were analyzed for major oxides and trace elements. Sample preparation for major oxides followed the fused disc method in which samples in powder are mixed with a lithium borate flux and then fused at 1100 °C in a muffle furnace in a Pt-Au crucible. The discs were then measured via X-ray fluorescence spectroscopy (XRF) using an X-ray Philips PW-2400 XRF spectrometer. For trace elements, multi-acid digestion was employed on powders and then the solutions were analyzed via inductively coupled plasma mass spectrometry (ICP-MS). Both analyses were performed at ALS laboratories (Medellin, Colombia) using the OREAS 24b standard (https:// www. oreas. com/ crm/ oreas-24b/). Loss on ignition was determined from the weighed samples, and iron was reported as total Fe 2 O 3 . The chemical data was plotted in the software GCDkit (Janoušek et al. 2006).

Geochronology
K/Ar dating was carried out at ActLabs (Ontario, Canada) after separating glassy groundmass portions (<1 mm). For the K analysis, the sample aliquot was weighed in a graphite crucible with lithium metaborate/tetraborate flux and fused using a LECO induction furnace. The fusion bead was then dissolved with acid. For the Ar analysis, sample aliquots were weighted in an Al container, and degassed at ~100 °C for 2 days.
Determination of radiogenic argon content was performed twice using an MI-1201 IG mass spectrometer with the isotope dilution method and 38 Ar as a spike. Two standards (P-207 Muscovite and 1/65 "Asia" rhyolite matrix) were measured for 38 Ar spike calibration (actla bs. com).
Field relationships indicate that Las Margaritas 2 sits on top of Las Margaritas 1. Las Margaritas 1 coulée comprises massive rocks, occasionally displaying spheroidal weathering patterns (Fig. 2a). In hand sample, the rock has a porphyritic texture with phenocrysts of amphibole, plagioclase, and biotite embedded in a gray glassy groundmass (Fig. 2b). Las Margaritas 2 lava flow comprises massive rocks with columnar jointing (Fig. 2c). The rocks have a porphyritic texture with phenocrysts of amphibole and plagioclase (but no biotite) embedded in a gray glassy groundmass (Fig. 2d). Las Margaritas 2 volcano contains lithic fragments of Las Margaritas 1 (Fig. 2e).
Ages obtained from the groundmass yielded 0.77 ± 0.04 and 0.80 ± 0.05 Ma, for Las Margaritas 1 and Las Margaritas 2, respectively (Table 1). These indicate that the volcanoes formed during the Pleistocene, with the two eruptive events being closely separated in time.

Petrography
The Las Margaritas 1 volcano is characterized by a porphyritic texture and contains 35 vol.% phenocrysts and microphenocrysts in a cryptocrystalline to microcrystalline groundmass of plagioclase, amphibole, and Fe-Ti oxides (Fig. 3a). Phenocrysts and microphenocrysts are of plagioclase (20 vol.%), amphibole (9 vol.%), and biotite (6 vol.%). Plagioclase is mainly euhedral (Fig. 3a), varies between 0.2 and 5.2 mm in size, and can often be found forming glomerocrysts with amphibole and biotite (Fig. 3a). Many plagioclase crystals display zonation (Fig. 3b) and a few show a sieve texture characterized by a dusty zone in the crystal rim (fine sieve) (Fig. 3c). Amphibole is euhedral to subhedral, varies between 0.2 and 1.4 mm in size, is oxidized, and is often found with resorption features (Fig. 3d) and poikilitic texture. Biotite is subhedral (Fig. 3a, d), and its size varies between 0.4 and 1.2 mm. Fe-Ti oxides also appear as inclusions in plagioclase and amphibole.

Whole-rock chemistry
Major oxides and trace elements are reported in Table 2. Both volcanoes are andesitic in composition with a calkalkaline signature (Fig. 5a-b), varying between medium and high potassium. Trace elements indicate an enrichment in light rare earth elements (LREE) in comparison to heavy rare earth elements (HREE) (Fig. 5c). Las Margaritas 2 has a higher enrichment in REE (Fig. 5c) and several other trace elements (Table 2) in comparison to Las Margaritas 1. Trace element profiles also show negative Th, Nb, Ta, and Ti anomalies, and a positive Sr anomaly in Las Margaritas 1 (Fig. 5d). Eu/Eu* values vary between 0.81 and 0.97, i.e., the samples display small negative Eu anomalies.

Discussion
Las Margaritas are two effusive and monogenetic centers (cf. Murcia and Nemeth 2020) with complex geometrical characteristics that could be related to the shape of the feeder vent, the duration of lava discharge (Mériaux et al. 2022), the shape of magma conduit, and the topography conditions (Murcia and Nemeth 2020).  Mapping indicates that the smaller Las Margaritas 1 couleé was erupted first, and the larger Las Margaritas 2 lava flow second (Fig. 2e). The K-Ar ages for the two volcanoes are similar and within error; thus, there is a possibility of polymagmatic monogenetic volcanism (cf. Brenna et al. 2010;Van Otterloo et al. 2014;Németh and Kereszturi 2015;Kugaenko and Volynets 2019) with two different magma batches ascending from the same reservoir to feed Las Margaritas 1 and 2, respectively. This would make Las Margaritas 1 and Las Margaritas 2 two units comprising a single monogenetic polymagmatic volcano. Apart from this, the ages highlight the existence of volcanism as recently as 0.8 Ma in a region where volcanism was not recognized until this study.

Petrography
Products from the volcanoes contain plagioclase, amphibole, and biotite as phenocrysts and microphenocrysts (Figs. 3a, 4a), although Las Margaritas 2 lacks biotite. Based on the mineral characteristics, it can be suggested that: (i) The larger crystals were formed in early crystallization stages under slow cooling conditions, low nucleation rates, and slow magma ascent (Cox et al. 1979;López and Bellos 2006), while smaller crystals were formed at more superficial levels, with more rapid cooling conditions, higher nucleation rates, and faster magma ascent (López and Bellos 2006). (ii) The microlites within the groundmass were related to the final stages of crystallization and the syn-eruptive phase, in which the magma undergoes fast decompression, degassing, and exsolution of H 2 O due to a rapid cooling (Renjith 2014).
Equilibrium textures in the minerals are apparent from aggregates of multiple interlocking euhedral crystals (both microphenocrysts and phenocrysts) forming glomerocrysts. This texture represents the aggregation of crystals during convection or stagnation processes (Vance 1969;Hogan 1993;Seaman 2000). Zonation in plagioclase is interpreted to be caused by convection driven by small-scale physicalchemical perturbations or by differential reaction between the crystal and the melt (Best 2003;López and Bellos 2006;Viccaro et al. 2010;Renjith 2014); this allows CaAl-NaSi diffusion within the crystals (Shcherbakov et al. 2010).
Disequilibrium textures in the minerals include the coarse sieve texture in plagioclase (Figs. 3c, 4a), which suggests decompression processes that caused dissolution; when an unaltered rim occurs, it indicates that the crystal ascended to a new equilibrium zone that allowed the formation of the new rim (Viccaro et al. 2010(Viccaro et al. , 2012Renjith 2014). Resorption textures in amphibole (Figs. 3d, 4c) are the result of melt incorporation in the crystals due to rapid cooling during magma ascent, or decompression processes (Gill 2011). Amphibole oxidation rims (Fig. 4c-d) were likely caused by the response to water loss during slow magma ascent due to the decompression processes (Rutherford and Hill 1993;Ridolfi et al. 2008). The poikilitic texture of plagioclase in amphibole formed because of the differential nucleation and growth rates (López and Bellos 2006). The amphibole's lower nucleation rate allowed it to grow more and incorporate a previously formed plagioclase crystal (Winter 2001;López and Bellos 2006). In addition, the lack of biotite in Las Margaritas 2 suggests a more rapid ascent in the superficial levels, preventing  (Castro and Dingwell 2009). The lack of amphibole oxidation rims in Las Margaritas 1, and the presence of them in Las Margaritas 2, can be related to fast ascent rates at deeper levels for Las Margaritas 1 (Hall et al. 2004;Ridolfi et al. 2008) that did not allow the reaction rims to form, but a slower ascent rate at superficial levels that allowed biotite to crystallize.
We suggest that the first stage of crystal formation happened at depth, involving equilibrium conditions between the crystals and the melt, with low nucleation and ascent rates, which allowed the phenocrysts to form. During this stage, the magma had different stagnation and small-scale convection processes that formed glomerocrysts and compositional zoning in the plagioclase phenocrysts. The second stage of crystallization happened at lower depths and was characterized by the formation of microphenocrysts related to higher nucleation and ascent rates. During this stage, convection processes caused zonation in the microphenocrysts. Disequilibrium textures were formed due to decompression, more rapid cooling, and a decrease in water concentration. The final stage happened near the surface and corresponds to the last stage of crystallization. This stage was characterized by decompression processes, very rapid cooling, and high nucleation rates that generated the microlites of the groundmass.

Las Margaritas volcanoes
The sub-alkaline andesitic composition with the medium K calc-alkaline affinity of the Las Margaritas volcanoes is typical Figure 5 Geochemical diagrams with data normalized volatile-free. A Total alkali vs. SiO 2 diagram (Le Bas et al. 1986). B AFM diagram (Irvine and Baragar 1971). C Rare earth elements (REE) normalized to chondrite (Nakamura 1974). D Extended trace element diagram normalized to chondrite (Sun 1980). As reference: Pijaos monogenetic volcanic field (PMVF; Velandia et al. 2021), Villamaría-Termales monogenetic volcanic field (VTMVF; Borrero et al. 2009;Botero-Gómez et al. 2018;Salazar-Muñoz et al. 2021), and Samaná monogenetic volcanic field (SMVF; Borrero et al. 2017;Monsalve et al. 2019;Sánchez-Torres et al. 2019 of subduction zone magmatism (e.g., Gill 1981;Best 2003;Rollinson and Pease 2021). The evolved character of the Las Margaritas magmas is evidenced not only by the SiO 2 contents (60-61 wt.%), but also by the low values of Fe 2 O 3 (~6 wt.%), MgO (3 wt.%), and CaO (5 -6 wt.%) in comparison to a primitive magma. Las Margaritas volcanoes show a general enrichment of large-ion lithophile elements (LILE) such as Ba, K, and Sr and a depletion of high-field strength elements (HFSE) such as Nb, Ta, Th, and Ti (Fig. 5d) . These anomalies can be related to the contribution (or not) of terrigenous subducted sediments (Pearce and Peate 1995) or differentiation processes during magma ascent (Rollinson 1993). Negative anomalies in Nb can also be related to clinopyroxene fractionation, while negative anomalies in Ti can be evidence for amphibole and Fe-Ti oxide fractionation (Fig. 5d) (Rollinson and Pease 2021). The negative Eu anomaly (Fig. 6) can indicate plagioclase crystallization at depth and posterior fractionation (Rollinson, 1993). In the multi-elemental diagrams (Fig. 5c-d), Las Margaritas 1 and 2 display similar trends; however, Las Margaritas 2 shows more enrichment, especially in the REE diagram (Fig. 5c). This enrichment can be explained by a lower degree of partial melting, since REE enrichment decreases as the percentage of melting increases (Winter 2001). The presence of biotite in Las Margaritas 1 is evidence of crystallization conditions at shallow levels and temperatures lower than 800°C (Rutherford and Devine 2003).

Las Margaritas volcanoes vs. other monogenetic fields
Las Margaritas volcanoes show similar geochemical behavior in comparison to monogenetic volcanic fields nearby. This is apparent from the positive correlations of K 2 O, Ba, and Rb vs. SiO 2 (Fig. 6). Furthermore, the negative correlations of MgO, V, and CaO/Al 2 O 3 vs. SiO 2 show the fractionation process of ferromagnesian minerals principally (Martínez et al. 2014). This is reinforced by the horizontal linear trend in the Nb/Th vs. SiO 2 diagram (Fig. 6). Fractional crystallization processes are also evident in the Yb/ Dy vs. SiO 2 diagram (Fig. 6), where the arrow direction mainly indicates an increase in amphibole fractionation (cf. Sánchez-Torres et al. 2022). Moreover, the positive Eu anomaly found in some volcanoes of the Samaná and Villamaría-Termales fields (Fig. 6) can indicate plagioclase accumulation at more superficial levels, while the negative Eu anomaly found in the Las Margaritas volcanoes can indicate plagioclase fractionation.
Compositional variations within each volcano demonstrate individual crystallization histories for each one. Yet they mostly behave as a group on the Nb/Th vs. Nb/Zr diagram, which can indicate that the magmas came from the same crustal magma reservoir (Gómez-Vasconcelos et al. 2020). Therefore, Las Margaritas volcanoes can be related to the magmatic accumulation zone in the crust (20-35 km depth) that has been proposed as feeding the SCVTP, based on seismic tomography (Londoño 2016;Murcia et al. 2019) and supported by geothermobarometric calculations (Salazar-Muñoz et al. 2021;Sánchez-Torres et al. 2022).

Figure 7
Structural geological models proposed for the Las Margaritas volcanoes. A Scheme representing the drainages structural control that were used to identify the kinematics. B Proposal 1: scheme representing a dilatancy zone formed by the interaction between the El Perro and Tapias faults, which allowed the magma emplacement. C Proposal 2: scheme representing an en-echelon system linked to the acting E-W compression in the region, which allowed the magma emplacement Reservoirs at lower crustal (33-36 km) or at mid-crustal levels (23-26 km) have also been proposed in other monogenetic fields, such as the Wulanhada Volcanic Field, northern China (Luo et al. 2022).

Fault controls
The volcanic zone where the Las Margaritas units were emplaced is controlled by the N-S Manizales fault and NE-SW El Perro fault system (Fig. 1c), which around the Las Margaritas volcanoes are classified as NE-trending reverse faults. There is also a clear E-W drainage trend in the area (Fig. 1c), which aligns with the E-W elongated morphology of the Margaritas volcanoes. Shear relations and drainage displacements associated can be used to propose the strike-slip movement of the faults in the Las Margaritas area (Fig. 7a). Displacement of the Guacaica river generated by the El Perro fault reveals sinistral movement, and curvature of the Manizales fault generated by the Tapias fault reveals the dextral movement of the latter (Fig. 1c). Thus, the interaction between the El Perro and Tapias faults may have generated a dilatant zone, which allowed magma ascent (Fig. 7b). Alternatively, an en-echelon fault array linked to the E-W compression in the region generated elongated fractures, with a 45° angle to the shear zone, thus allowing magma ascent (Fig. 7c).

Conclusion
Las Margaritas 1 and 2 are two newly identified monogenetic eruptive centers in the Manizales region of Colombia that add to the list of monogenetic effusive activity involving evolved compositions that are little studied worldwide. In particular, our study highlights the existence of Quaternary volcanism (~800 ka) in a location where it was not recognized prior to this study. The reason why these magmas erupted effusively is still unclear, while mineral textures and the evolved composition suggest fractionation related to magma stagnation. This shows that monogenetic volcanoes are formed not only by rapid ascent of small batches of magma with primitive compositions, but also by magmas taking off from crustal reservoirs.